In accord with the varying food distribution systems and dietary habits in recent years, the food packaging style and mode have been strikingly changing, which in turn has created an increasing demands toward high quality wrapping films. A degraded quality of food, which is caused by temperature, moisture, oxygen, ultraviolet rays, microorganisms such as bacteria and mold, and the like during distribution and sales of the food, leads to serious problems of loss of sales and insufficient food hygiene. Such degradation of quality has been conventionally eliminated by adding an antioxidant, a preservative and the like directly to food. In view of increasing levels of strict regulation imposed on food additives for the protection of consumers, however, the amount allowable to be added is inevitably decreasing.
Under the circumstances, a demand has been rising toward a wrapping film having smaller vapor or moisture permeability and which does not degrade food value due to freezing, boiling treatment, retort treatment and the like.
To be specific, when wrapping fish meat, meat, shellfish and the like, oxidation and spoilage of protein, fats and oils, and the like need to be inhibited but taste and freshness should be retained. This requirement is met by cutting off air permeation by the use of a wrapping material having superior gas barrier property. In addition, a gas barrier film used for wrapping food contributes to the retention of flavor of the content and inhibition of moisture permeation. As a result, a dry food is not deteriorated by the absorption of moisture and a water-containing food is not spoiled or solidified by the evaporation of moisture, thus enabling retention of flavor for an extended period of time, which was fresh at the time of wrapping.
For the above-mentioned reasons, it is considered extremely important that a wrapping film used for, for example, a food made from boiled fish paste such as boiled fish sausage, dairy products such as butter and cheese, fermented soybean paste, tea, coffee, ham and sausage, instant food, confectionery such as castella and biscuit, and the like, have oxygen barrier property, moisture-proof property and flavor retaining property. These properties are not to be fulfilled only by a film for wrapping food but are very important for a film used for wrapping medical products that require handling in a sterilized state and electronic parts that require rust-proof property.
As a film having superior oxygen barrier property, moisture-proof property and flavor retention, there have been known, for example, a film wherein a metal such as aluminum is laminated on a plastic film, and a film wherein vinylidene chloride or ethylene vinyl alcohol copolymer is coated.
While the above-mentioned aluminum laminate is superior in economic aspect, oxygen barrier property, moisture-proof property and flavor retention, it is associated with a problem that opacity thereof hides contents after wrapping and that it cannot be placed in a microwave oven, since the film does not pass microwaves.
In addition, a film coated with vinylidene chloride or ethylene vinyl alcohol copolymer has only insufficient gas barrier property against water vapor, oxygen and the like, and shows marked degradation of the property by a high temperature treatment. The film coated with vinylidene chloride generates chlorine gas on incineration, and the adverse influence exerted thereby on the global environment has been a major concern.
There has been known a gas barrier film wherein an inorganic thin layer of silicon oxide or aluminum oxide is formed on a plastic film. For example, Japanese Patent Examined Publication No. 48511/1976 proposes a gas barrier film comprising SixOy (e.g., SiO.sub.2) layer deposited on a synthetic resin film as a gas barrier film permitting clear view of the content and use in a microwave oven. However, SiOx (x=1.3-1.8) having superior gas barrier property is slightly brown, and has insufficient transparency.
Japanese Patent Unexamined Publication No. 101428/1987 proposes, as a transparent gas barrier film, a film comprising a layer made from aluminum oxide in the main which is formed on a plastic film. However, the film has insufficient gas barrier property and insufficient flexibility (specifically poor endurance to the gelbo treatment).
A conventional laminate film having an inorganic deposited layer of silica, alumina and the like does not necessarily have sufficient film strength, and poses problems of degraded oxygen barrier property and moisture-proof property after boiling treatment, retort treatment and the like. An inorganic deposited layer of silica, alumina and the like is frequently formed on a polyester film (PET). For example, when a film has a construction of, for example, PET/deposited layer/adhesive layer/oriented nylon (ONY)/adhesive layer/undrawn polypropylene (CPP), the shrinkage of nylon brings about degraded oxygen barrier property and moisture-proof property after boiling treatment, retort treatment and the like. To overcome this problem, a construction of PET/deposited layer/adhesive layer/PET/adhesive layer/undrawn polypropylene (CPP) is commonly employed. This construction nevertheless has only insufficient strength to bear a shock produced by dropping same. As described above, a laminate film having an inorganic deposited layer has been associated with difficulty in accomplishing film strength, oxygen barrier property and moisture-proof property after boiling, retort treatment and the like.
Moreover, there has been proposed films comprising an oriented nylon (Japanese Patent Examined Publication No. 12649/1995) as a substrate to be used for deposition, or a nylon as a laminated layer (Japanese Patent Unexamined Publication No. 276571/1995), both nylons imparting strength to the proposed films. However, neither of them is suitable for practical use, since production and storage during transportation thereof are complicated. Japanese Patent Examined Publication No. 12649/1995 proposes a nylon having smaller shrinkage percentage during high temperature treatment, wherein the shrinkage percentage which is the added absolute values of dimensional variation in the machine direction and transverse direction, said variation being caused by heat treatment at 120.degree. C. for 5 minutes, is not more than 2%. Yet, boiling treatment using hot water at high temperature and retort treatment cause an increase in shrinkage percentage of nylon, thus failing to maintain fine oxygen barrier property and moisture-proof property.
Japanese Patent Unexamined Publication No. 276571/1995 proposes a film comprising an oriented nylon having a smaller shrinkage percentage in hot water, said nylon being laminated on an inorganic deposited layer of a film comprising the inorganic deposited layer and a substrate layer. However, the nylon layer showing a low shrinkage needs to be laminated besides a sealant layer and the process becomes complicated to increase production cost.